This invention relates to an improved design for seating hard wearing button inserts in interfering sized cavities in percussion drill bits and will be described with particular reference thereto. It is to be appreciated, however, that the invention has broader applications and may be adapted to use in a number of other environments.
Button inserts formed from sintered carbide or other hard materials normally mounted in generally cylindrical cavities in drill bits with one end of the inserts protruding therefrom. The other or inner ends of the inserts are seated against the bottom surface of the associated cavity. During operation, a percussion tool strikes the top of the drill bit. The impact stress waves caused by this percussion travel through the drill bit to the inserts which, in turn, fracture the rock against which the drill is held. As a result of this action, considerable impact forces are generated during the drilling process.
The seating surface between each insert and the associated cavity in a drill bit is the major area for the energy transmission of these impact forces with resultant severe stress concentrations therebetween. These stress concentrations are due to the difference in elasticity between the carbide of the insert (100,000,000 psi) and the bit material (30,000,000 psi). The stress concentrations are also due to the interplay of the manufacturing tolerances for the drill bit and insert, and, eventually, failure results.
In the prior art, great concern has been focused on corner stress concentrations in, and the subsequent fatigue failures of, button insert drills. In some cases some sort of captured shape has been recommended to confine the forces, or a corner clearance has been used to minimize the magnitude of the developed corner forces. These solutions have merit as long as there is no angular manufacturing tolerance deviation between the adjoining contacting surfaces of the insert inner end and the bottom of the cavity, ie., the inner end surface of the insert and the bottom surface of the cavity meet with full planar contact. As soon as there is some sort of angular deviation between these adjoining surfaces, the surfaces will no longer meet in a plane, and the angular stress concentrations will overwhelm the proposed prior art solutions and result in numerous drill failures. Moreover, to limit this angular deviation, the manufacturing tolerances must be tightly controlled in these types of prior art devices.
The present invention overcomes the foregoing problems and others to provide a new and improved insert seat arrangement. The invention successfully compensates for any manufacturing tolerance angular deviation between the adjoining contacting surfaces and minimizes uneven stress concentrations in the drill.